text:
What constitutes an "emergency" in flight procedures?
In reading through the more informal literature (Wikipedia, FAA PDFs, Av.SE), I often come across a caveat to standard flight rules and procedures: "unless an emergency exists". The intention is obvious; ATC is going to prioritize helping an aircraft that encounters a situation that simply cannot be ignored. However, most such resources don't go into specifics about what kind of situations constitute an "emergency". There are obvious emergency scenarios, like a "dead stick", and there are obvious non-emergencies like "I have to be on the ground by 5:00 to beat traffic home". Then there are grey areas like "I fought a headwind the whole way here, I'm low on gas and now my home field is IFR and won't grant me SVFR. I might make it to my alternate even further upwind that's clear, but I really think it's best to set down here even if I can't see the far end of the runway".
So, the questions:

What kinds of things constitute a genuine emergency, and what will get a pilot laughed at by ATC (or worse)? Where's the line?
Is ATC allowed to judge what is or is not an actual emergency, or must they prioritize any aircraft declaring one?
Does fault or blame matter at the time the emergency is declared? Will ATC de-prioritize a situation that is obviously the pilot's fault, like flying into the only cloud for 50 miles in any direction?
Is there anything you still can't do as PIC in an emergency situation? Apparently in an emergency you can do a lot of things that would get your license pulled in any other scenario, like violating separation minimums, entering airspace that's normally off-limits like R-zones and Class A space, running on instruments without a rating, etc. While these things are still dangerous, it is apparently the pilot's prerogative to resolve the emergency however he can. Is there anything a plane declaring an emergency still must do other than fly the plane, or conversely something he still cannot do like get too close to Air Force One?
After declaring an emergency in flight, what should be done after the plane's back on the ground (hopefully safely)?

I'm putting this as a comment as I retired in 1999 and things may have changed since. ATC will take seriously any declaration that a pilot is using his emergency authority. However, regardless of the emergency, the FAA will want to talk to you when you're on the ground, and certificate action can follow if regulations were broken without cause. There is a "careless and reckless" reg that was often used as a catch all. A good test as to whether it's actually an emergency insofar as aircraft problems is concerned is whether it required a landing at the nearest suitable airport.

There is a really good explanation of emergency situation over at the SkyBrary:

An emergency situation is one in which the safety of the aircraft or of persons on board or on the ground is endangered for any reason.
An abnormal situation is one in which it is no longer possiible to continue the flight using normal procedures but the safety of the aircraft or persons on board or on the ground is not in danger.
Emergency or abnormal situations may develop as a result of one or more factors within or outside an aircraft, for example:

Fire on board the aircraft;
Aircraft component failure or malfunction (e.g. engine failure, landing gear malfunction or loss of pressurisation);
Shortage of fuel (or other essential consumable substance);
Flight crew uncertain of position;
Worsening weather;
Pilot incapacitation (e.g. as a result of illness);
Aircraft damage (e.g. as a result of collision, bird strike or extreme weather;
Illegal activity (e.g. bomb-threat, wilful damage or hi-jacking).

So in short, anything that puts the flight in danger can be considered an emergency.

Is ATC allowed to judge what is or is not an actual emergency, or must they prioritize any aircraft declaring one?

If the pilot declares emergency, ATC will provide assistance and support to the flight without judging whether it is an actual emergency or not. ATC cannot know in most cases, nor are they really required to analyse the situation to come to that conclusion. Their whole responsibility is to ensure the flight receives all required information and support to either end the abnormal/emergency situation or land at a suitable airport.

Does fault or blame matter at the time the emergency is declared? Will ATC de-prioritize a situation that is obviously the pilot's fault, like flying into the only cloud for 50 miles in any direction?

No, they will not de-prioritize based on the type of emergency. Even if a pilot flew VFR into IMC conditions, he is still in a life-threatening situation from which he needs to recover, so assigning blame or guilt is not helpful here, nor would that be the ATC's job. This is the Civil Aviation Authorities' responsibility, once the flight is back in safe conditions. ATC may file a report, though, but not during the emergency, but as an aftermath.

Is there anything you still can't do as PIC in an emergency situation? Apparently in an emergency you can do a lot of things that would get your license pulled in any other scenario, like violating separation minimums, entering airspace that's normally off-limits like R-zones and Class A space, running on instruments without a rating, etc. While these things are still dangerous, it is apparently the pilot's prerogative to resolve the emergency however he can. Is there anything a plane declaring an emergency still must do other than fly the plane, or conversely something he still cannot do like get too close to Air Force One?

Technically, once you have declared an emergency, you can do whatever is required to keep the flight safe and prepare for an emergency landing. If you need to enter otherwise controlled or restricted airspace, this will be made possible by ATC via coordination with adjacent units. Without knowing FAA regulations on this specifically, I would daresay that distress traffic could have priority over the Air Force One.

After declaring an emergency in flight, what should be done after the plane's back on the ground (hopefully safely)?

Unless your CAA or their respective investigative service is not already waiting for you, you should remain with your aircraft (unless being treated for medical issues and taken to a hospital) and wait for them to arrive. If in doubt, you can call the police and they will notify the relevant authorities.

Do you differentiate between "emergency" and "abnormal situation"/"urgency" when talking to ATC? Do you "declare an abnormal situation"?

@KeithS, urgency is declared with "pan-pan-pan" (same form as "may day" for emergency otherwise) or "declaring urgency". Abnormal situation is a generic term for anything, that is not normal and is not declared that way.

The regulation for the PIC Authority after declaring an emergency (which would be good to quote in this case) is 91.3(b) and says: "In an in-flight emergency requiring immediate action, the pilot in command may deviate from any rule of this part to the extent required to meet that emergency". They can't do anything....

Also, unless there is an accident or incident (NTSB 830 definition), I doubt that the FAA will meet you after landing just because an emergency was declared (you may be waiting a LONG time, lol).  In fact, there is a registered which covers this and says that you must submit a written report if requested...

WITH DUE RESPECT I DIFFER A BIT
I am an ICAO controller. We classify flight status as follows:

Normal= Self-explanatory.
Abnormal: Abnormal reading/performance but no danger present yet- time for anticipation
Local Standby (urgency FAA): potential safety issue of the aircraft or of persons on board and requires timely assistance.
Full emergency (distress FAA)= An emergency situation is one in which the safety of the aircraft or of persons on board or on the ground is endangered for any reason. A full emergency is imminent danger and requires immediate assistance.

On the other hand, we should:

GET INFO Unless clearly stated by the flight crew, take all necessary steps to ascertain aircraft identification and type, the type of emergency, the intentions of the flight crew as well as the position and level of the aircraft in order to handle the emergency intelligently
SEPARATE: Upon receipt of information that an aircraft is having an emergency, all possible action shall be taken immediately to safeguard all aircraft concerned.
Establish/maintain separation with other aircraft and terrain!
ASSIST: Decide and deliver upon the most appropriate type of assistance which can be rendered
COORDINATE: The air traffic controller concerned shall inform and enlist the aid of any other air traffic controllers and control sectors which may be affected.

To respond to an emergency, we  (AIR TRAFFIC CONTROLLERS - ATCOs) should demonstrate problem-detection, anticipation of threats, planning, decision-making, resilience, error management, task distribution, teamwork and performance abilities. Re-planning is part of the ATCOs duties since, as an occurrence evolves over time, new threats may appear whilst current threats may change their demands.

Where exactly does that differ from my answer? I think it's saying quite the same, with more detail on the controller perspective.

After having a problem that I did not declare an emergency, a very wise 15,000hr airline pilot friend told me there is a very good rule of thumb - I now live by it.
"An emergency is anytime the weather or aircraft does not operate as intended or expected that jeopardizes the safety of flight."
Later I ran across some FAA information of interest. (Aim pg 220) For ATC purposes (maneuvering), a pilot is required to notify ATC whenever their aircraft is unable to climb or descend at least +-500fpm. An airplane that can not meet those requirements is not necessarily an emergency but is a "problem".


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How can I get 'lead point' for aircraft to intercept final course?
As controller, I always wonder when should I instruct aircraft to turn. The aircraft does not turn at right angles as soon as instructed, but rather has turning radius. So, I tried to find the formula but failed.
If speed of aircraft is 300 kts, and he is instructed to make 90 degree, how long distance the aircraft need to turn? I make image, and I wonder the formula for 'x'

Is that 300 kts airspeed or groundspeed? At what altitude does the aircraft make the turn?

I didn't consider those things as factor. For exeample, altitude is 5,000ft and ground speed. And I also want to know the fomula for 30degree or other random degree case. Thanks.

I was taught the following rule of thumb for providing a final vector to final, in the USA, using the STARS radar system which (when using ADS-B inputs) has a one-second update time:
Set the Predicted Track Line to 30 seconds. Vector the aircraft to a 90º base-to-final. When the PTL reaches the extended runway final, issue the Position-Turn-Altitude-Clearance. The aircraft should roll out on a good heading to intercept final approach course.
This is obviously at low altitudes and relatively low speeds, 230ish knots or below. Theoretically (because the method uses the aircraft's speed instead of a fixed distance) this method should work for any aircraft going any speed, but that may not always pan out. In particular, some military aircraft will be flying rather quickly on the base leg and slow down as soon as they get their dogleg turn-to-final, which could end up leaving them too far from final for a legal intercept.

At low speeds, aircraft fly specific turn rate, so it works. But at higher speeds they fly specific bank angle, and then the turn radius grows with square of speed, while the predicted track only grows with speed linearly.

Standard turn rate is 360° in 2 minutes so a 90° turn will take half a minute (30 s). The distance covered will be $ s = \frac {v}{60 \times 2} = \frac {v}{120} $ (nautical miles).
The number you are looking for is the radius of the arc. This is given by $ s \times \frac {2}{\pi} $ (nautical miles).
So the radius is given by $ r =  \frac {v}{120}\times \frac {2}{\pi} = 0.0053 v$. This is approximately $ \frac {v}{200} $.
So for 300 kt the start turning would be $ \frac {300}{200} = 1.5 $ nautical miles from rollout.

I'm an engineer, not a pilot or controller, so wait for confirmation of my maths by someone who knows what they're talking about.

I think your math is correct, but an aircraft traveling that fast under IFR will generally make half standard rate turns to avoid excessive angles of bank.  The rule of thumb we used for a half standard radius was 1.0% of airspeed.  So, 3nm at 300kias, 2.5nm at 250kias, etc.  Avoids algebra while flying!  ;)

That makes sense. You can post it as an answer. Thank you.

Thanks for answer. If the degree is 30, Could I get the lead point too?

@Transistor, I don't have time to verify against your formulas, but feel free to incorporate into your answer to make it more complete.

@Min, I will assume you are asking degrees of turn and not angle of bank.  I am not sure if the math above supports this, but personally I would just divide by 3.  For example, if I was doing 300 knots and using my rule of thumb of 3nm for a 90 degree turn, I would lead a 30 degree turn by about a mile.   I would also recommend you confer with fellow controllers for advice on this matter.

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text:
Where can I find the exact definition of VFR and IFR?
Although I have worked as ATC more than 5 years(not US), I don't know the exact definition of VFR and IFR.
Of coure, I know that under VFR, pilot mainly use their eyes to navigate, but I want to know more exact and offical reference.
I tried to search these via google but failed to find offical reference such as ICAO annex or FAA regulations.
Could you guys help me to find the offical reference relating to VFR and IFR?

In Who does have a responsibility to make altitude correction for cold temperature?, you say you're in Asia; in What device in western countries allows ATC to get clearances or search flight plans automatically?, you seems familiar with the situation in Korea. But where are you located? "Not US" covers an awful lot of Earth surface area, and if you want the official definition, then telling us which official definition you want probably helps.

Related: How can pilots fly inside a cloud?

The Rules of the Air are defined by ICAO and made applicable with few deviations in the ICAO member states.
ICAO Annex 2 Rules of the Air, defines in addition to the general rules, the Visual Flight Rules (VFR) and the Instrument Flight Rules (IFR). Flights shall be in compliance with either of them.

2.2 Compliance with the rules of the air
The operation of an aircraft either in flight or on the movement area of an aerodrome shall be in
compliance with the general rules and, in addition, when in flight,
either with:
a) the visual flight rules; or
b) the instrument flight
rules.

Chapter 5 of Annex 2 contains the Instrument Flight rules.
Chapter 4 of Annex 2 contains the Visual Flight Rules.

4.1 Except when operating as a special VFR flight, VFR flights shall be conducted so that the aircraft is flown in conditions of visibility and distance from clouds equal to or greater than those specified in Table 3-1.

Table 3-1 is summarized below.
+---------------------------+-------------------+---------------------+-----------------------------+
|      Altitude band        |   Airspace class  |  Flight visibility  |     Distance from cloud     |
+---------------------------+-------------------+---------------------+-----------------------------+
| At and above 3 050 m      | A*** B C D E F G  |        8 km         | 1 500 m horizontally        |
| (10 000 ft) AMSL          |                   |                     | 300 m (1 000 ft) vertically |
+---------------------------+-------------------+---------------------+-----------------------------+
| Below 3 050 m (10 000 ft) | A*** B C D E F G  |        5 km         | 1 500 m horizontally        |
| AMSL and above 900 m      |                   |                     | 300 m (1 000 ft) vertically |
| (3 000 ft) AMSL, or above |                   |                     |                             |
| 300 m (1 000 ft) above    |                   |                     |                             |
| terrain, whichever is the |                   |                     |                             |
| higher                    |                   |                     |                             |
+---------------------------+-------------------+---------------------+-----------------------------+
| At and below 900 m        |   A*** B C D E    |        5 km         | 1 500 m horizontally        |
| (3 000 ft) AMSL, or 300 m |                   |                     | 300 m (1 000 ft) vertically |
| (1 000 ft) above terrain, +-------------------+---------------------+-----------------------------+
| whichever is the higher   |        F G        |        5 km**       | Clear of cloud and with the |
|                           |                   |                     | surface in sight            |
+---------------------------+-------------------+---------------------+-----------------------------+
* When the height of the transition altitude is lower than 3 050 m (10 000 ft) AMSL, FL 100 should be
  used in lieu of 10 000 ft.
** When so prescribed by the appropriate ATS authority:
    a) flight visibilities reduced to not less than 1 500 m may be permitted for flights operating:
        1) at speeds that, in the prevailing visibility, will give adequate opportunity to observe
           other traffic or any obstacles in time to avoid collision; or
        2) in circumstances in which the probability of encounters with other traffic would normally
           be low, e.g. in areas of low volume traffic and for aerial work at low levels.
    b) HELICOPTERS may be permitted to operate in less than 1 500 m flight visibility, if manoeuvred
       at a speed that will give adequate opportunity to observe other traffic or any obstacles in
       time to avoid collision.
*** The VMC minima in Class A airspace are included for guidance to pilots and do not imply
    acceptance of VFR flights in Class A airspace

For Air Traffic Management purposes, ICAO Doc 4444 gives more detail of how both types of flights are treated by ATC.

National regulations may deviate from ICAO regulation. If they do deviate, the deviation have to be published in the Aeronautical Information Publication of the country.

VFR is a combination of being in good conditions ( having 3SM visibility, and staying 500/1000/2000 ft clear of clouds), and being responsible for your own navigation, terrain clearance and traffic-avoidance (see-and-avoid).
Anytime you're not operating under VFR rules in VMC conditions, you are either IFR or operating illegally!
You can be following IFR rules even when conditions are good.
You cannot be VFR when you have an open IFR plan.

"In order to operate under Visual Flight Rules, visual meteorological
  conditions (3 Statute Miles (SM) visibility and at least 1,000'
  ceilings) must exist In addition to operating within VMC conditions,
  pilots must maintain specific distances from clouds, depending on
  airspace [Figure 1]"

https://www.cfinotebook.net/notebook/rules-and-regulations/special-vfr

Your answer is incomplete. Any time the weather is not VMC you must be on an IFR flight plan. You may also be operating on IFR rules if you are on an IFR flight plan, regardless of the weather conditions. Then there is VFR-on-Top where you are operating on a combination of VFR rules and IFR rules. And see-and-avoid applies whenever conditions allow it—not just when flying VFR.

There is also Special VFR.

In the US, the definition of VFR depends on which airspace you are operating in. The rules for VMC (Visual Meteorological Conditions) are listed in FAR 91.155. The definition of IFR is easy. If conditions are less than the minimums listed for the type of airspace, you are in IFR conditions.

There is no such thing as "IFR conditions"; alternatively, it could refer to both VMC and IMC (so would be a meaningless term). There's VMC, and there's less than VMC minimums (which is IMC). In VMC, a pilot may elect to fly either VFR or IFR, at convenience; in IMC, it is prohibited to fly VFR, so only IFR flight is allowed.

You negated your own argument above in your paragraph. IFR conditions are precisely what that means, and what you state, meteorological conditions under which only flight under instrument flight rules can be conducted. If you wish to disagree, that's fine, take it up with the FAA.
"Step 1 – Review Weather Minimums. The
regulations define weather flight conditions for
visual flight rules (VFR) and instrument flight rules
(IFR) in terms of specific values for ceiling and
visibility. " https://www.faa.gov/news/safety_briefing/2015/media/SE_Topic_15_02.pdf

The grammatical hang up for many (including myself) results from putting the word "conditions" behind IFR instead of in front of it.  "Conditions (meteo) under which IFR (rules) must be followed" is much more clear than "IFR Conditions".  By phrasing it the latter, it appears as if the conditions refer to a set of rules rather than to the weather.  Although everyone knows what is meant by "IFR Conditions", I agree that this phrase is incorrect -- there is no such thing. But even official docs are written by humans, and "IFR Conditions" is found in numerous places throughout FAA material.

As a member of the grammar police, sworn to serve and correct, I have submitted many change suggestions through the official FAA process whenever I find this incorrect phrase in an FAA document.

As long as the difference in terms is clearly understood I can't see anyone getting hung up on the difference between "conditions under which IFR must be followed" and "IFR conditions".  The latter is simply a cleaner way to say the same thing.  I occasionally get frustrated when the terms are intermingled, but tolerate it when context is clear and I believe the person knows the difference.  Once I intervened on the radio, when my co-pilot reported we were VFR when in fact we were operating IFR in VMC.

Except that phraseology exists for a reason. "When context is clear" is too subjective. For example, just look at the title of 91.151: "Fuel requirements for flight in VFR conditions".  Your co-pilot (and probably others) understood your flight context to be operating "in VFR conditions", since this phrase is frequently (and incorrectly) substituted to mean VMC. However, he would be in violation if he planned his "IFR in VMC" flight to 91.151 standards instead of 91.167 standards. I firmly believe these two paragraph titles must be changed (and many other locations in many other docs).

And just to be clear... I am well aware that the term "IFR Conditions" is listed and defined in 14 CFR Part 1: IFR conditions means weather conditions below the minimum for flight under visual flight rules.  (I still think it's a bad and confusing term)

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Approach when weather condition is below minimum

If airport weather condition is below minimum, but pilot report that flight weather condition is above minimum, Does controller can issue approach clearance to pilot?
If weather condition get worse than published minimum, should pilot cancel his approach and execute missed approach even in final approach phase?

I'm a little confused. You are taking about IFR concepts, but weather minimums. If the pilot gets to the MDA without establishing visual, they have to go around and execute the missed approach. That's purely a pilot decision. If you are talking VFR, they don't execute missed approaches...

@RonBeyer  The question make much better sense in a Part 121 or 135 world where wx below mins = can't start the approach. The answer by wbeard52 not only answers the question, but helps one make sense of what's being asked.

You haven't told us what country you are referring to.  I am going to talk about the United States regulations.
There are two separate regulations that come into play.  Part 91 and Part 121 or 135.
Under Part 91.  The pilots are allowed to commence any approach (irrespective of weather) and determine if they can continue to land based on the criteria listed in §91.175(c).
Under Part 121 / 135.  The pilots can only commence an approach (proceed past the final approach fix) if the latest weather received is above landing minimums.  Mainly, pilots are concerned with visibility.  They will not be able to use "flight visibility" as the weather visibility is most likely different as the airplane descends towards the ground.  The pilots must use the weather reported from the airport.
To answer the second question, if the pilots are past the final approach fix and new weather arrives indicating the airport is below minimums, the pilots are allowed to continue the approach to minimums to see if they can land per §91.175.

Thanks for answer. My country refers to ICAO regulations. Is it different with FAA?

There are many places where ATC doesn't give clearance at all. A VFR flight can be completed without any ATC communications of any kind.
Assuming an ATC clearance is required, ATC doesn't give an approach clearance based on reported weather, they give a clearance and it is up to the pilot to fly the approach and execute a go around according to the approach procedure if visual contact with the runway or approach lights isn't established. Often weather conditions are changeable, what was reported in the last METAR can be significantly better or worse just a few minutes later, so a pilot can give the approach a try.
With very few exceptions a pilot can go around at any time, even on a late final. From a weather point of view it is a judgment call to be made by the pilot, if visual contact with the runway is lost for example. Notable exceptions I can think of are airfields like Aspen, where a go-around is impossible after a certain point in the approach.

Thanks for answer. I understand that you mean ATC don't have authority not to give approach clearance even weateher condition is worse than published weather condition. If so, I wonder why there is minima criteria for IAP. Regardless of minimum of IAP, can pilot decide to land if he have viaual reference at MAP/DH?

Depending on the jurisdiction, there may be legal implications if the pilot is caught continuing the approach when the ceiling and/or visibility is below reported minimums.


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What degree is crosswind?
I know that there are three type of wind, headwind, tailwind and crosswind.
What degree of wind direction can distinguish it? 30 or 45 degree?

So you want to know how many degrees of crosswind component qualifies a wind to officially be called a "crosswind"?  You won't find that any where in the FARs or AIM, so I guess it's not a real thing.

I'm not sure we can speak in degrees. I think we usually divide the wind in 2 components (one on the axis of whatever you consider --route, runway,...-- and another perpendicular) and speak of cross-wind component. crosswind would be if the sideway component is above a threshold.

Ah I just thought of the real answer-- if there's enough sideways wind component that we should land with a  sideslip rather than a forward slip, then it counts as a crosswind.

We should probably look at wind as only having two types, crosswind and wind in our direction of travel. Headwind and tailwind are the same type. One is the inverse of the other. Or, you can say that one is the negative and one is the positive of the same type. Otherwise, why wouldn’t you say that there are four types of wind: headwind, tailwind, left crosswind, and right crosswind? And, breaking it down further into quartering winds would confuse matters even more by adding four more types. It is more correct to say that there is only one type broken into two components for our practical use.

Manu H has given what, to me, sounds like as close to a textbook answer as you can get.
Crosswind is always 90° to your direction of travel. You base crosswind off of your track instead of your heading. Regardless of your track or heading, or the wind direction, the crosswind intensity would be a percentage of the total wind intensity ranging from 0% to 100% coming from 90° to your direction of travel. You can find the numeric value of the crosswind using Pythagorean’s Theorem. The headwind/tailwind component and the crosswind  component make up two of the legs of the mathematical 90° triangle. The actual wind bearing from your direction of travel and its intensity make up the hypotenuse.
Or, you can use the graph, below:

FYI, a good rule of thumb (that you can see in the graph) is to treat any crosswind above 60° as the same as 90° and for less than 60° use a “clock" estimate. e.g 15° is 1/4 of an hour so the crosswind is 1/4 of the wind velocity. 30° is 1/2 of an hour so the crosswind is 50% of the wind velocity.

@JScarry Never heard that technique before, but that is tremendously cool. Pity I can't award a bounty to a comment - that one would qualify!

Any wind that is not perfectly parallel to the runway is a crosswind, because any non-parallel wind has always a 90º component, big or small...


text:
What's the relation between ADS-B and MODE S
I know that ADS-B uses 1090ES or UAT to transmit their signal, and 1090 ES is related to MODE S.
Here's my questions.

What's the 1090 ES?

If a aircraft uses 1090 ES as their ADS-B out, Does aircraft must equip with MODE S?

Does every MODE S have 1090ES functions?

The 'S' of Mode S is 'selective', but it doesn't match with ADS-B that broadcast without interrogation signal. Can you explain about it?

Your questions answered shortly

What's the 1090 ES?

SSR protocol is used to manage transmissions on SSR frequencies (1030 and 1090 MHz). Mode S is a part of this protocol to manage data exchange between interrogator and transponder. 1090ES is a part of mode S describing how a transponder can transmit long unsolicited messages (extended squitters) which are used to carry ADS-B message (DF17/DF18).

Layers involved in ATCRBS and ADS-B transmission, source

If a aircraft uses 1090 ES as their ADS-B out, Does aircraft must equip with MODE S?

Yes, 1090ES is a user of mode S. But ADS-B is not required to use a transponder to broadcast the ADS-B message, it can use UAT (or VDL) as well.

Does every MODE S have 1090ES functions?

No, Some older Mode-S transponders do not support 1090ES, they cannot send long messages required to broadcast ADS data.

The 'S' of Mode S is 'selective', but it doesn't match with ADS-B that broadcast without interrogation signal.

Selective interrogation is used by the transponder to filter incoming interrogations. ADS-B is an 'broadcast' service, messages are send automatically using the transponder (so the name 'squitter'). From a transponder standpoint, messages are not replies to interrogations, even if they use 1090 MHz, the downlink frequency to reply and to broadcast.
More details follow if you're interested.

SSR/Mode S and ADS-B technologies, like PBN/RNAV, are not well treated in technical literature due to the numerous changes during their infancy. The result is the wording is often misused and the big picture is blurry, only unrelated details are discussed without providing the whole context.
The key point is to identify clearly, like in OSI networking model, the different functional layers and data formats used. ADS-B and SSR Mode S are not on par, ADS-B is a higher level service using the lower transmission capabilities of mode S.
And indeed ADS-B has nothing to do with SSR reply to interrogation also found in Mode S, but driven by a different higher service.
Mode S concept
Mode S specifications include two aspects:

The possibly to identify an aircraft using SSR: This part is compatible with modes A/C. Mode A and C transmissions have simple formats with fixed information sent embedded in 12 SSR pulses.

The possibility to exchange data: Mode S is bidirectional, it defines data frames to transmit or receive data with a varying format and content. This is actually a rudimentary network protocol. A mode S transponder is able to receive and send these varying formats. ADS-B messages are sent using this data transfer capability.

1090ES out is a layer within the transponder which transmits only. 1090ES is capable of assembling a long message (112 bits). In this scheme ADS-B logic provide the data to be transmitted. The way transponders work is to use 'registers'. When it's time to send a mode S frame, the transponder use the data available in the registers to assemble the frame, then sends it. So ADS-B store data to be transmitted in the transponder registers, and 1090ES layer assembles the frame, known as 'extended squitter'. 'Squitter' because the transponder sends it without prior interrogation (broadcast), 'extended' because its length is 112 bits instead of 56.
What is involved when a ADS-B transmission occurs
Data preparation:

DF17 (or DF18 when the message is not to be sent by a transponder), the format of a message carrying ADS-B data
ADS-B logic feeds the mode S transponder registers relevant to ADS-B with data.

Data transmission, if using SSR:

1090ES allowing long (112 bit) mode S messages (extended squitters)
1090ES compatible mode S transponder to send DF17 messages on SSR frequency using the extended squitter feature and data in the registers.

ADS-B message generation

SSR interrogation
An ATCRBS interrogator on the ground can interrogate all transponders (A/C/S) using multiple combinations to select either only mode A/C, or S, or a combination. It can also elicit replies from all mode S transponders, or from a single one. This is this selective interrogation function you refer to in your question.
Mode S reply versus mode S broadcast
Responding to ATCRBS is usually done in mode S by the same logic which is at work in mode A and C, except an additional 56 bit data frame is added.
In addition mode S transponder have the possibility to broadcast frames without prior solicitation, this is the squitter mode.
Basic data level: Mode S data format
Mode S is based on a 56 bit data frame which can be extended to 112 bits. Generally what can be inserted in this frame is defined in ICAO Annex 10, there are 25 different frame content (formats) used for uplink (UF formats for reception by the transponder) and 25 formats for downlink (DF formats for transmission by the transponder).
Among these downlink formats there are two important ones: DF11 is used to reply to 'all call' interrogation (equivalent to replies in mode A and C), and DF17 which includes additional information, among them the position, is used for ADS-B out.
Short vs long data frames
A mode S transponder can broadcast two types of unsolicited messages: Short (56 bits) and long (112 bits). Initial versions of mode S transponders were designed to manage messages short messages. This was compatible with the DF11 message used as acquisition squitter for TCAS and to reply to Mode-S acquisition interrogations (UF11).

Mode S downlink formats currently in use
The long format, usually referred to as the extended squitter is the one used to squitter DF17 message used by ADS-B.
1090ES refers to a long message transmitted on 1090 MHz, the frequency used by mode S to (reply and) broadcast.
This name 'squitter' comes from the DME transponders. The ground station used in DME is a transponder which replies to aircraft interrogators. In order to transmit something without interrogation, and allow aircraft to synchronize on the DME station, these transponders were sending random pulses called squitters. Mode-S reused this terminology to refer to any unsolicited transmission.
One level up: The ADS-B service
ADS-B is a user of the transponder. How ADS-B is materialized is actually undefined, but it's usually a piece of software in the transponder box, or a separate box. It builds DF17 messages and asks the Mode-S transponder to sent them using the Mode-S protocol.
The chain is somehow similar to using a modem to communicate:

ADS-B is the application creating DF17 messages to be sent like a web browser creates HTTP messages.
DF17 messages are encapsulated in squitters, like HTTP messages are encapsulated in IP packets.
Squitters are transmitted by a mode S transponder like IP packets are transmitted using a modem.

ADS-B (1090ES variety) does not need to be on a Mode-S transponder. It is possible (and foreseen in t standard) to develop non-transponder based ADS-B devices. However, in the vast majority of cases, 1090ES ADS-B will be transmitted from a Mode-S transponder.
DF17 messages cannot be sent over UAT or VDLm4. DF (Downlink Format) 17 is part of the Mode S specification. The other technologies (UAT, VDLm4) have their own separate formats, which are very different.

Dear mins, I made a few edits to your post to correct some misunderstandings. There are a few things that I am not sure about (e.g. the part where you claim that initial mode S transponders were designed for only 56 bits. I am quite sure 112 bits messages have been part of the spec since the beginning). I left them as is.

@DeltaLima: Thanks, I appreciate and trust your changes. I'll try to add references, but as you know the existing documentation is very difficult to use and I was not able to find good overviews.

What is available online is often of poor quality and the official standards are quite expensive. Unfortunately I can't share them with you. Nevertheless, you made a good answer!

@DeltaLima. After verification I confirm you're right, the 112 bit frame was included early in the design of Mode S (present in an old 2006 draft of DO-181D available online).

Mode-S transponder and TCAS II developments are closely related. TCAS uses both 56 bit (DF0) and 112 bit messages (DF16) in it as well. IIRC, the long ones are needed during resolution advisories, so I guessed the 112 bits formats were there from the very beginning.

What's the 1090 ES?

1090MHz is the frequency used for replies by Mode A/C/S transponders when they are interrogated on 1030MHz.
ES stands for “Extended Squitter”.
Extended refers to supporting 112-bit data frames in addition to Mode S’s original 56-bit data frames.
A squitter is a transponder that sends replies without being interrogated. This is a weird concept, but it’s central to how ADS-B works within the framework of Mode S.

If a aircraft uses 1090 ES as their ADS-B out, Does aircraft must equip with MODE S?

1090ES is a superset of Mode S, i.e. it does everything that Mode S does plus ADS-B.

Does every MODE S have 1090ES functions?

No. All older Mode S transponders must be replaced with new 1090ES units to comply with ADS-B mandates. However, many manufacturers offer trade-in/upgrade programs.

The 'S' of Mode S is 'selective', but it doesn't match with ADS-B that broadcast without interrogation signal. Can you explain about it?

The general idea is that Mode S was already widely adopted (at least for turbine aircraft) and well understood, and they could get the desired functionality for ADS-B just by adding a few extra data frame types, which was a lot less work (and cost to most operators) than building an entirely new parallel system.


text:
Does 'RADAR' include MLAT or ADS-B, etc?
Nowadays, There are many surveillance systems other than PSR or SSR, such as ADS-B or MLAT, etc. And ATC system such as ARTS uses various surveillance sensors including ADS-B to indicate track for controller.
But the 'RADAR' means 'Radio detection and ranging'. And I have doubt as to whether this can be included radar category because ADS-B broadcast position of aircraft by using GNSS.
And these are my questions.

Is ADS-B or MLAT RADAR?

When ATC identify aircraft by using ADS-B only, Does controller can say 'Radar contact'?

Can SSR indicate the aircraft position?

Related: What is the difference between SSR and ADS-B?

Is ADS-B or MLAT RADAR?

It depends on context, unfortunately. For the purposes of your questions below, the answer is “yes”, as technically wrong as that may be.

When ATC identify aircraft by using ADS-B only, Does controller can say 'Radar contact'?

In the US, yes, because that’s currently the only approved phraseology for controllers to use, even if the contact is not actually from radar.
The ICAO equivalent is “identified”, and the FAA has been gradually coming into compliance with ICAO phraseology, so it’s possible that this will change in the future.
Also, the FAA’s Fusion software takes data feeds from many sources (PSR, SSR, ADS-B, MLAT, etc.) and combines them all into a single display, so the controller may not be aware of which source(s) is(are) providing the target on their screen; all contacts appear to be “radar” contacts.

Can SSR indicate the aircraft position?

Yes. SSR works similarly to primary radar, with a beam sweeping the sky; the two are often combined into a single unit. However, rather than looking for a reflection from the aircraft’s skin, SSR looks for a reply from the aircraft’s transponder. Either way, the azimuth and range of the reply is then translated to a lat/long position.

From a terminology standpoint ADS-B is not "radio detection" because it does not search for a target, as PSR/ASR does, but rather waits for the target to announce itself to the receiver. Normal ADS-B is not "radio ranging" either, but you could make an argument that multilateration is. Kind of stretches the definition though, IMO.

Is ADS-B or MLAT RADAR?

No, Automatic Dependant Surveillance Broadcast (ADS-B) and multi-lateration (MLAT) (or the wide-area variety of multi-lateration, WAM) are not radar. They are, like radar, ATC surveillance technologies.

When ATC identify aircraft by using ADS-B only, Does controller can
say 'Radar contact'?

That is probably depending on the country. But what is important to understand is that the controller is telling the pilot by 'Radar contact' that they can track where you are based on some kind of surveillance system, and that they can give you instructions based on that position information. To the pilot and air traffic controller, the underlying surveillance technology is of secondary importance.

Can SSR indicate the aircraft position?

Yes, like Primary Surveillance Radar (PSR), Secondary Surveillance Radar (SSR) determines the bearing and range from the radar to the aircraft. In addition it receives, encoded in the reply from the transponder, the altitude (in mode C or Mode S). With this information the position of the aircraft is determined.


text:
What are the operational differences between the ILS CATs?
I know ILS CAT is classified by DH, and RVR, but I want to know what's difference between ILS CAT operationally.
For example, CAT 3 approach should be autolanding. (I am not sure)
And, if RVR is 600m which is CAT 1 RVR, Do pilot can make CAT 2 approach or should pilot make CAT 1 approach?

If you are asking the difference between two things you need to mention what both of them are in the title. If instead you are asking different types of CAT then that would sound like a 'search it' problem.

Related: How do you choose CAT II vs. CATIII ILS?

Also related: What is the purpose of having different ILS categories?


text:
What are the merits of an inlet cone/spike/centerbody vs. a 2D inlet?
And which is better for a scramjet?
Is a 2D inlet basically a "2D cone"? Similar to how an aerospike engine is made "2D" in the linear aerospike?
What's inside the cone/spike? Or is it just hollow?

Both are ways to create compression shocks in order to compress and decelerate supersonic air.
Spike intakes
The central spike is characterized by:

undisturbed flow,
light weight and compactness, and
easy adjustability to speed and mass flow at high flight speeds by translating the spike.

Its disadvantages are:

large loss of efficiency with angle of attack and sideslip,
long internal ducting,
limited space for radar installation, and
limited mass flow at low speed.

Typical central spike intakes are those of the MiG-21 or the SR-71, as shown below. Note the variation in cone angle which helps to create additional, steeper shocks such that the flow is gradually decelerated over a sequence of shocks. The inside is filled with a radar system or hollow in case of the SR-71. At the height of the intake lip the SR-71 spike has a perforated wall to suck away the spike's boundary layer, and aft of that it contains the translation mechanism which shifts it back and forth to adjust the intake cross section and to adjust the shock position.

A variant of the central spike is a side intake with a compression half-cone, as used on the Mirage III, the F-104 Starfighter or the TSR-2. Their location at the fuselage side helps to reduce the length of internal ducting and to reduce flow distortion due to sideslip. It also frees up space at the fuselage tip for a larger radar antenna. However, now the fuselage boundary layer has to be split off by a splitter plate to ensure equal flow energy over the whole intake cross section.
The picture below shows the English Electric Lightning with its central spike intake feeding two Rolls-Royce Avon engines, and the side intake of the TSR-2. Note the additional intake doors aft of the intake lip of the TSR-2 which were needed to increase mass flow at low speed.

The F-111 has a quarter-circle spike intake. Here, the wing root also helps to direct the flow, so angle-of-attack variations produce less intake flow distortion. To suck away the intake boundary layer, the spike is perforated at the lengthwise position of the intake lips.

Ramp intakes
What you call 2D-intakes are normally rectangular with an oblique shape. The forward edge is used to create an oblique shock which can be positioned to hit the rearward edge on the opposite side by shifting one or both edges up and down. This also changes the cross section of the intake, so adjustments for mass flow can be made over a wide range. An adjustable ramp aft of the forward edge allows to create more shocks with decreasing obliqueness, such that the flow is decelerated by a sequence of shocks. Typical ramp intakes are those of the F-14, the F-15 or the Concorde. Their advantages are:

Adaptability over a wide range of flow conditions, angles of attack and mass flows, and
tailoring of the shock system for optimum efficiency over a wide range of supersonic speeds.

Their disadvantage is a consequence of the wide adaptability:

High weight and complexity.

Note that the pressure in the intake is a multiple of outside pressure at supersonic speed, so the intake is actually a pressurized duct. A rectangular intake will need massive structural reinforcements to stay in shape while the cross section transitions to a round shape downstream.
The drawing below shows the intake of the F-14 at different flow conditions. Note that the sharp intake lips require them to be exactly aligned with the local flow direction to avoid flow separation.

While a central spike intake is the best choice for flying at the design speed, the rectangular intake works better over a wide range of speeds. The shift in air combat doctrine from flying as fast as possible (1950s) to flying mostly subsonic with brief supersonic dashes (since the 1970s) is reflected in the choice of intake type.
Hypersonic intakes
When flying at Mach 5 and above, the obliqueness of the shock waves will need a very long intake, and effective designs use the lower side of the forward fuselage as part of the intake to create the necessary sequence of shocks for pressure recovery. Now the fuselage nose becomes the forward lip of the intake, and the body is shaped to create ever steeper shocks. Alternatively, the lower forward fuselage is shaped to work like a section of a spike. What looks like the actual intake is actually the rear intake lip, and the divergent section of the intake aft of the rear lip is reduced since the flow in the combustion area is still supersonic in order to keep temperatures manageable, making the engine a scramjet. This answer has more information on supersonic combustion.
Note that this type of intake is optimized for one speed, and very little adjustment is possible. The pictures below show a NASA CFD study of the X-43A (left, source) and the Russian Ajax project (right, source). A scramjet can use both types of intake, and the fuselage design will determine which type it will be.

The rear fuselage now becomes the nozzle of the engine, because most of the expansion will happen after the flow leaves the enclosed part of the engine.

Fantastic! Exactly what I was looking for. Just a couple quick questions: 1) Mounting the spike on the side of the fuselage reduces the length of the internal ducting. Why? Because the air doesn't have to flow from nose to tail?

2) Why does the fuselage-mounted spike perform better than nose/wing mounted ones? Because the fuselages acts like a 'ramp extension' and directs flow into the intake?

3) What's the difference between an inlet and intake? Just semantics?

4) You said ramp intakes carry a mass and complexity penalty. Can you characterize that in [approximately ballpark] kg and USD? If an F135 weighs 1.8 tonnes and costs USD 18 million, how does the intake compare?

5) Since the spike is small/cramped, can you enlarge the internal volume by added a parallel/cylindrical midsection [without changing the shape/curve of the face/tail]? Or does that interfere with the flow?

5b) Could you integrate a very modestly sized turbine/compressor into the spike to increase mass flow at low speed, eg taxiing+takeoff? For example, if you were designing a NASA GTX -esque air-augmented rocket, and you didn't want to run in pure-rocket mode from takeoff until the ram effect takes over.

Let me know if you'd like me to post these as separate questions for you to answer. I will most certainly, happily, and expediently do so. I've been trying to teach myself a smattering of aerodynamics, and this community has been so incredibly helpful... Speaking of which, is there anything I can do as a way to say thanks and give back to the community?

@HephaestusAetnaean: 1) Exactly!, 2) It doesn't perform better, but gives the best overall compromise. The ramp extension is something you get with an F-16 type intake, 3) Not sure. For me an inlet is smaller, an intake is the real thing, 4) No, really. 5) Small is beautiful. Why would you do that? 5b) Compressor suction is enough, and for low speed you add doors around the intake.

@HephaestusAetnaean: My answer to your last question: Upvote. Help others find the good answers. When you learn enough, share as well. That might take years, but do not feel pressured.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
For a teardrop shape, which orientation has the least drag?
Here's the shape:

(Well, that's a cross section of the shape. The actual object is a 3D shape obtained by rotating that outline around the y-axis.)
Let's pretend it's a solid, rigid object flying through the air. It can fly point-first or curve-first. Which way produces the least drag?
Assume it's normal air at room temperature. The angle of attack will be constant. It will be subsonic speeds.
BTW, is there a huge difference in drag? If not then it doesn't matter much.

Although aerodynamics is a big part of this site it should be something that relates to aviation. A non-aviation related aerodynamics question like this is better in physics.se

VTC since the 3D shape, as described, does not appear to be about aviation, specifically, and would fit better in physics.se

Why did someone VTC to meta.aviation?

I'm voting to close this question as off-topic because Belongs on Physics.se

Look at this familiar object, this is a practical use and an answer. Beware of imitations.

The design looks similar to the wheel fairings found on many Cessna-type aircraft. They lead curve-first.
If it was going tip-first, you could have a nice laminar airflow initially, but once it reaches the curve, the airflow will separate from the surface and produce a big wake with associated drag. By leading with the curve you can have a laminar flow right to the tip, with only a little bit of drag.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Can a turbofan have a fan driven electrically so as to run at optimal speed?
I just watched this pretty dumbed-down video about the idea of putting a gearbox in a turbofan so the fan can run at its optimum slower speed while the core runs at its optimum high speed.
And I immediately remembered this answer from Peter Kampf that said basically, a normal gearbox would have huge waste heat.
Then I suddenly remembered an old question of mine about driving a helicopter engine with a diesel-electric system.
So now the obvious 2-and-2 together: Can the fan of a turbofan be driven with an electric motor (powered by the already-existing generator of the engine)? Or the more detailed questions: Can an electric motor be small enough to fit around the fan's axle and still power the fan? How much it would weigh? I'm interested in large turbofans on the orders of 100 to 500 kN, but I think that would make the task easier since there's more room plus weight increases have diminishing returns against a huge airliner.
This would have obvious advantages. The fan:core ratio can be variable without any gearing. Presumably much less maintenance, or at least no more maintenance than the generator requires.

Electric transmission is only useful when mechanical transmission is not feasible, e.g. WWI era tanks and WW2 era ships. Currently mechanical transmission has the advantage in terms of weight, space, and energy efficiency. But things becomes interesting if electricity is generated by mechanical generator, e.g. solar panel.

"Can the fan be driven with an electric motor " Yes, in principle, but you are replacing a simple steel shaft with a 50MW electrical generator and a 50MW motor, and except at takeoff both those would only be running at say 20% of their power rating - but you still have to carry all the extra weight around. Plus you need more bearings to support two separate shafts, more control systems to match up the electrical power generated and used, etc, etc. FWIW the P&W geared fan is an engine option for the A320neo, but airlines aren't interested - it's too unreliable.

@alephzero Can you explain how you got a value of 50 MW? Seems pretty big to me. Boeing 777 engine does around 500 kN max, so not even 1 meganewton. And remember the fan will be spinning slower than normal to prevent supersonic tip speed.

@DrZ214: Power (watts) != force (newtons).  Some basic math tells me that 500 kN (engine force) * 100 m/s (air speed) = 50 MW (engine power).  Power is the product of force and speed.

There are plans to run a turbofan-like engine with an electric motor rather than the gas generator used today. But the weight of electric engines does not promise an immediate advantage - only when the electricity is generated in a much more efficient way that what today's gas generators offer will the overall propulsion mass be lower. An example is a hydrogen-powered fuel cell somewhere in the fuselage.
We had a similar discussion here and here.

Can you cite exactly how much power the fan alone will need during takeoff and during cruise? I'm thinking for a plane like the Boeing 777, which I choose just because it's still got the biggest turbofans of an airliner. If I know what power rating the motor needs, I can look up how big and heavy such a motor will be.

@DrZ214: No, not exactly. We had a similar question here

RR's book "The Jet Engine" p101 says the fan provides 75% of the engine thrust... so knowing a 777's thrust and the formula that equates lbs thrust to eqv hp, you'll get your answer.


text:
Can a jet engine benefit from nitrous oxide boost?
I just read this wiki article again about the GM-1 boost system. This was used in Nazi planes during WW2. It injected Nitrous Oxide (N2O) into the supercharger of the piston engine. The idea was to get more oxidizer in the engine, allowing for higher fuel-injection and therefore higher engine power temporarily. This could serve to climb faster or take-off better or maintain high altitude.
My original interpretation of the GM-1 system and the related MW-50 system is that they're sorta like afterburners of their era. But that analogy is not perfect. Afterburners use the same jet fuel as used inside the engine, but sprayed outside the back. GM-1 used a different fuel, N2O (actually a monopropellant with more oxidizer built into the chemical than fuel), injected inside the engine.
What I want to know is, can N2O boost work in a jet engine by injecting directly into the combustion chamber (along with increased fuel injection)? Is that a good idea for military jet aircraft (compared to afterburners)? Is it even possible without overheating something?
I'm interested in turbojets and turbofans, and would even consider a centrifugal jet engine.

It would leave everyone giggling after a flyover.

I recall that the Tu-160 adds some nitrogen compound to its fuel (which causes the exhaust to look more orange). I can't find a good source for this though, so I don't know the details of what exactly they put in.

@Thomas I've never heard that before. If you find something, please come back and post it.

For the combustor, I say the answer is no, because what is limiting the fuel rate, isn't the amount of oxygen (gas turbine combustors run very lean), but the ability of the turbine materials to withstand a higher gas temperature entering the turbine. Hence, to temporarily increase engine thust, water injection into the combustor was used on early jet engines, to increase thrust during takeoff.
This sounds quite counter intuitive, but the explanation is based on thermodynamics. Naturally, the water evaporates into steam, which then gets heated to the temperature of the combustor exit temperature. This evaporation and heating process of the water and then steam, is drawn from the heat of the combustor, and so reduces the combustor temperature (for the same fuel flow). However, the engine does not need to add more fuel to make up for this cooling, at the same level of thrust, because the energy added to the steam is still available in the turbine, to be released as the combustion gas / steam mixture flows through the turbine, expands and cools. As it cools, it gives up this energy, which is effectively used to power the turbine. The remaining energy is then converted to thrust, as the flow expands through the final propelling nozzle. But now, because the turbine temperature is lower for the same thrust level, the fuel flow can be increased until the maximum turbine inlet temperature is reached, and so now providing more thrust.
So, rather than inject N2O into the main combustor, inject water.
For the afterburner (AB) however, the case is quite different, and I believe a performance benefit could well exist. AB combustion runs much closer to an equivalence ratio of 1 (about 0.96), i.e. burning almost 100% of the fuel level that can occur with complete combustion of all the available air. And unlike the main combustor, there are no real material temperature limits (cool bypass air exits along the inside of the AB duct surface). But, the question is, do you really want to do this, operationally? Is it better to use some of the planes tank capacity for some N2O, or just entirely for fuel? I suspect operationally it is better to be able to use the AB for longer, at a slightly lower thrust level, than shorter, for a higher thrust. I don't believe military aircraft use AB to out run missiles, so pure thrust isn't the only aim. But that's another (next) question....

I remember for a few high speed applications, e.g. sr71, the limitations sometimes is compressor temperature.

Well, the turbine inlet temp is always higher than the compressor exit temp, otherwise we have put no fuel in the engine, but compressor exit temps can get very high, and sometimes the last stages of the compressor uses different materials than the earlier stages so they can handle those conditions.

Adding nitrous to an afterburner is literally just making a rocket.

Several answers have mentioned injecting water into the engine. Th injection of water is not so much to lower temperatures, but to increase the density of the air moving through the engine. The denser the air the higher the trust on turbojet engines. I worked on KC135a and B52g aircraft in the USAF, both used water injection on take off. if the water ran out during take off, they had a difficult time getting into the air, especially on a hot summer day when the air density was low due to low humidity.

Power can be augmented in turbojets through water injection or afterburning as the other answers explain. Each technique has its advantages and disadvantages but neither is well suited for more than a few minutes -- typically during takeoff, climb, and war emergency power application during combat.
Water injection works best at low levels where there is more available oxygen. Afterburners work at all altitudes but make inefficient use of fuel compared to the base turbojet (afterburning specific fuel consumption is about twice as much per pound of thrust). The only jet engines that I know combine both techniques are in the MiG-25 and -31.
There are at present no jet engines that advertise nitrous oxide injection but that doesn't mean it couldn't be useful in certain situations. The first decision is whether to inject liquid or gaseous nitrous: liquid is clearly better both because of denser storage and phase change charge cooling. This cooling effect is the only reason why you might use nitrous at takeoff, but since you'd be adding oxidizer there would still be temperature concerns. The better place to use nitrous would be at altitude for extra climb performance or speed dashing where the engine would have excess available fuel (up to sea level flows). In such instances nitrous injection would probably be preferable to afterburning because maximum thrust would be greater (nitrous can produce up to 70 percent more power compared to afterburning's 50 percent) AND it would produce that extra power without afterburning's gas-guzzling specific fuel consumption. That means not only going faster but also going further. So there's definitely a place for nitrous injection in jets.
But what would Putin do? H would add nitrous to the water injection and afterburners on his MiG-31s and MAYBE get an airplane that's finally faster than an SR-71.

Nitrous works well in a piston gas engine because the mixture is in a rather narrow range, and additional oxygen can be utilized with a mixture that gets enrichened, plus has added oxygen.  In effect it creates the equivalent of an inlet boost, because it adds available oxygen.
The turbine engine runs much leaner, and has a wide mixture range which supports combustion.  Since turbines normally have extra oxygen, they would not benefit from the addition of available oxygen. At least not the way that a piston engine with a enrichened fuel mixture would.
So the net effect of injecting nitrous into a jet engine would be very small, and at max power settings, it would still be small due to the inherent lean operation and associated oversupply of oxygen in the combustion chamber.  In other words, adding oxygen when there is already plenty there provides little benefit.

And most to all of that benefit would be from the thermal decomposition of nitrous oxide into nitrogen and oxygen with the attendant increase in volume, not from the extra-oxidiser effect.


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What is the "Brake to Exit" feature on the Boeing 777X?
I have come across a new feature called BTE (Brake to Exit) on the 777X, but I am unable to find any documentation related to that. There are vendors who are working on this technology for the 777X.

Hint: watch the spelling: searching for "brake..." instead of "break..." will give much better results ;-)

@PerlDuck I dunno. After the 737 MAX debacle "Break to exit" does sound like it might be a new Boeing feature...

The feature is as follows: during high-speed taxi, the pilots remove their seatbelts and brake sharply. They then exit the plane via the windshields.

It is a system that Airbus already uses in the A380
and some A350's. They call it BTV (Brake to Vacate).
It allows the pilot to select a certain runway exit in advance (e.g. while approaching).
After touchdown, the plane automatically brakes so it can vacate the runway at that given exit.
Airbus says:

When the pilot chooses a runway exit point, the system indicates the estimated runway occupancy time and the minimum turnaround time. During the subsequent landing phase, and according to encountered runway conditions (i.e. 'wet' or 'dry'), the aircraft's deceleration is automatically regulated so it reaches the chosen exit at the correct speed.

(emphasis mine)
The BTE system by Boeing is a similar system with a different name. A related article in Avionics International about the B777X states:

“We have also added new safety features like optimal runway exiting and brake-to-exit, which will allow a pilot to tie an airplane’s rollout and stopping distance to a specific runway exit,” said Kirk Scarbrough, Boeing’s 777X systems chief engineer.

See also

What is Brake To Vacate (BTV)?
FAST magazine from Airbus, issue #44(PDF, 10MB) from July 2009, section "Brake-to-Vacate system", pages 17-25

thanks for the info. Any idea if it helps while landing in cross winds to do the course correction?

@NitinG I can only tell for Airbus' BTV. It doesn't do anything actively while the plane is still in the air and subject to crosswinds. It just announces to the pilots that the desired exit can or cannot be achieved via the PFD and an aural message ("RWY TOO SHORT"). Only when the plane has actually touched down it will apply the wheel brakes (automatically) and possibly tell (read: yell at) the pilot to apply reverse thrust if needed. It doesn't do any course correction while airborne, it just hits the wheel brakes.

@NitinG, It is just a slightly smarter autobrake that you tell the point where you want to be down to taxi speed and it automatically selects the deceleration rate according to how much distance remains at the moment it activates after touch-down.

BTV was optional for the A-380 and standard for the A-350.  Additionally, the A-380 had a selector knob to select BTV or Conventional Auto Brakes.  We do not have that selector on the A-350.  We simply have the choice of BTV or Auto Brakes Medium.  I don't recall seeing anyone ever use anything other than BTV.  It works very well.  Former A-350/330 Captain


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Do all B777 family aircraft have the same engines?
I think there are multiple variants of engines for the different variants in B777 family but I am not sure but is it GE90-115B in one while Trent and PW in others?

An aircraft designer should consider at least 2 different engine suppliers per model/variant. To tie the aircraft to one supplier would create a monopoly which would enable them to charge what they liked. Also, buyers may have a preference for a specific engine supplier.

TL;DR: No.

For the true answer refer to the FAA Type Certificate Data Sheet
777-200
2 Pratt and Whitney Turbofan Model:
PW4074, PW4074D, PW4077, PW4077D,PW4090,PW4084D, and PW4090-3
(Engine Type Certificate No. E46NE)
2 General Electric Turbofan Model: GE90-76B, GE90-85B, GE90-90B, GE90-94B
(Engine Type Certificate No. E00049EN)
2 Rolls-Royce Turbofan Model: RB211-Trent 875-17, RB211-Trent, 877-17,
RB211-Trent 884-17, RB211-Trent 892-17, or RB211 Trent 892B-17, RB211 Trent 895-17
(Engine Type Certificate E00050EN)
777-300
2 Rolls-Royce Turbofan Model: RB211-Trent 884-17, RB211-Trent 884B-17,
or RB211-Trent 892-17
(Engine Type Certificate E00050EN)
2 Pratt & Whitney Turbofan Model: PW4090, PW4098
(Engine Type Certificate E46N)
777-300ER
2 General Electric Turbofan Model: GE90-115B
(Engine Type Certificate No. E00049EN)
777-200LR
2 General Electric Turbofan Model: GE90-110B1
2 General Electric Turbofan Model: GE90-115B
(Engine Type Certificate No. E00049EN)
777F
2 General Electric Turbofan Model: GE90-110B1
2 General Electric Turbofan Model: GE90-115B
(Engine Type Certificate No. E00049EN)

From a related answer:

The Boeing 777-300ER is always equipped with General Electric
  GE90-115BL1 engines, while the baseline -300 can be ordered with four
  different engine models from General Electric, Pratt& Whitney or Rolls
  Royce. The Emirates Boeing 777-300 aircraft have the Rolls Royce RR
  Trent 892 engines installed. They deliver 415 kN of thrust, almost 100
  kN less than the GE90-115BL1 engines.

When a customer orders a Boeing 777-300 they can pick their choice of the four engines available for that model. When they order a Boeing 777-300ER they will get the GE engine.

So, airlines can order any aircraft with any compatible model?

Basically yes. The manufacturer decides what engine models they support. The fewer models, the fewer cost for the manufacturer. But also less choice for the customer who might want to have an engine from a specific manufacturer.

Note that according to Wikipedia, nobody has ordered a Boeing 777-300 since before 2006; the 777-300ER has effectively superseded it.

@NitinG The -200 and -300 are no longer in production. AFAIK, the -200ER, -200LR, and -300ER (and the F) are still in production, but 100% of the outstanding orders for current models are for the -300ER or F. The -200, -200ER, and -300 were the first generation of 777s, while the -300ER and -200LR are the second generation. The most popular models, by far, were the -200ER from the first generation and the -300ER from the second generation. The next-generation 777 already has well more orders than the -200, -300, and -200LR put together and it's still a few years from production.

Though @DeltaLima has already answered your question, I'd like to source my information from Wikipedia:
The following engine options are available for various 777 variants:

777-200    : PW4077 , RR877 , GE90-77B
777-200ER  : PW 4090 , RR 895, GE90-94B
777-200LR  : GE90-110B1 , GE90-115B1
777 Freighter: GE90-110B1 , GE90-115B1
777-300    : PW 4098 , RR 892 , GE90-92B/-94B
777-300ER  : GE90-115B1
777-8X     : GE9X
777-9X     : GE9X

The Wikipedia page appears not to understand the engine naming systems used by the three companies. "GE90-xxx" and "PWxxxx" are correct, (apart from the erroneous spaces after "PW") but there is no such thing as an "RR877" etc. Presumably the page author didn't realize that PW and GE are not just abbreviations for the manufacturer's names, but part of the engine names themselves. Rolls-Royce engine names do not follow that system.

Too late to edit my previous comment - a more correct "short" name for the RR engines would be Trent 877, Trent 895, etc., if it doesn't matter exactly which version of the 877 you are referring to.

@alephzero You're right, of course, though I'm guessing Wiki put RR there for people not familiar enough with the manufacturers to know that Trent engines are made by RR. "RR Trent 877" would probably have been a more correct way to list it while still making the manufacturer obvious, though. It is Wiki... you could always just fix it. :)

Interestingly, as Sports Racer's answer notes, the GE90 isn't even listed as an engine option for the -300 on the 777 type certificate. The orders and deliveries page for the 777 on WIki also looks like only RR or P&W engines were ever ordered for it.


text:
How to read this step climb profile? FL STEPS CYUL/0330/TUDEP/0340/NETKI/0350
How to read this step climb profile?
FL STEPS CYUL/0330/TUDEP/0340/NETKI/0350
I know CYUL is departing point but what do 0330 represents? The altitude in Feet?

FL330 is the initial cruising altitude after departing CYUL. At TUDEP, a climb to FL340 is planned, and at NETKI, there is another climb to FL350.

FL330 is not 33,000 ft (or at least, will rarely be 33,000 ft)

@Simon Should've put pressure altitude there.


text:
How is vibration isolation achieved in helicopters?
In many helicopters, they say they have reduced the vibrations significantly, but how is this isolation being achieved?

There are many techniques.
Passive reduction techniques involve designing the powerplant, transmission, gearbox and particularly the rotor(s) to minimize vibration. Designers carefully arrange all the moving parts to be as closely balanced as possible. They can be mounted using flexible parts and springs to damp vibrations (the same principle behind the shock absorbers in an automobile). The sophistication of helicopter design has evolved over time as engineers learn more about the characteristics of helicopters and how mechanical parts behave. Computer-assisted design has also allowed for huge improvements here as moving parts can be simulated in great detail to optimize each individual part for minimum vibration.
Active systems work by using accelerometers to sense the vibration of the helicopter at certain points in real time. This data is then used to shake a reaction mass back and forth to create vibration opposite to that of the rest of the helicopter. The two opposite vibrations cancel each other out. Effectively, the active system is using electrical power to dump all of the vibration energy in the helicopter into a small mass, so the helicopter stays still while the reaction mass in the active system shakes to absorb the energy. Such systems are already in common use in large transport helicopters and are becoming more prevalent in smaller helicopters as well.

Despite all that, some helicopters still vibrate significantly. Some helicopters, mainly designed and built by Eurocopter/Airbus can be adjusted to be flying without vibration (EC155, EC225, EC175 for instance).
One of the main items to get right is the so-called rotor-track and balance (RTB). Older or smaller helicopters need specific maintenance for RTB using a stroboscope to see if all the blades follow the same track. This can only be done on the ground.
Some helicopters have a camera-system fitted that is able to capture the track of each blade. A computer system knows which blade is in the camera view and upon return can make a recommendation how to adjust the trim-tabs. This can only be done by day and won't work under specific lighting conditions (camera pointing too much into the light for instance).
Some of the newest helicopters don't even have a camera anymore, but accelerometers on the gearbox and the computer knows the position of each blade and calculates the necessary adjustments to be made.
If the RTB is done right, the active anti vibration system has little work to do. This is rarely done to perfection as most operators accept it when it is within 'manufacturer tolerances', so there still is vibration. To get it perfect costs time (and money).

A stroboscope?  Huh, kids today!  I have covered the end of the blade tips with different coloured chalk, then walked forward with a "flag" until the blade tips hit the flag and left 4 lines of different colour. You could then see which blade(s) need adjustment and complete.  http://encyclopedia2.thefreedictionary.com/flag+tracking


text:
At what intervals do planes land and take off from Chicago O'Hare?
Is there some data available which shows the intervals at which planes take off and land at Chicago O'Hare?
Just by taking a look around the last time I visited ORD couple of months back, it seems to be one almost every 10-15 secs.

The arrival and departure rates vary (mainly based on weather) - Are you looking for a rough average, or under specific conditions / at a specific time of day?

mainly looking for a general average for a month/year? I know the travel seasons or weather impacts it still, there might be some average which generally goes in.

@NitinG: For big hubs like ORD almost all traffic is scheduled airline traffic, so you can get good estimate by simply looking at the schedule.

As a general average, Chicago O'Hare (KORD) seems to do somewhere between 1.5 and 2 operations per minute (so roughly one takeoff or landing every 30-40 seconds).
I based this on data from the FAA's Air Traffic Activity Data System - Terminal Operations report, and specifically looked at November 23 through November 30 of 2014 (17835 operations, over 8 days works out to 1.55 operations per minute - I poked around randomly and found days as high as 1.7 operations per minute).
One of the limiting factors for number of operations at any airport is separation between aircraft, which is covered here

Incidentally we were discussing airport/tower activity levels in chat earlier today, and Chicago O'Hare is the busiest airport in the United States overall - a title it recently stole back from ATL (which was #1 from 2004 to 2013).

Actually, ORD has not been busiest in terms of operations until mid 2014... ATL held the title for the previous ~10 years I believe.

@slookabill Dang, you're right! Fixed :-) (it's been one of the two for as long as I've paid attention, but I thought O'Hare had been on top for the last few years for some reason)

@voretaq7 You could also change "in the United States" to "in the world," though either is technically correct. :) Beijing at #5 and Heathrow at #10 are the only non-U.S. airports in the top 10 worldwide when measured by movements. ATL is still tops by pax throughput (by a wide margin.)


text:
Do pilots need special training to land Lockheed U-2 reconnaissance aircraft?
Do pilots need special training to land Lockheed U-2 reconnaissance aircraft as its body is too shallow and is a wide wing aircraft?

All military pilots will receive special training when learning to fly a new aircraft, whether that is in a simulator, a 2 seater version of the same aircraft, or both. An F-16 pilot is not going to fly a F-15 without considerable training. Training is recommended even for light aircraft as well when learning a new model.
If you are asking whether the U2 is harder to land than many other airplanes then yes, the U2 is a very challenging aircraft to land because of it's shape, but also because the pilot can see very little of the runway. In fact the procedure for landing a U2 calls for a vehicle to speed down the runway behind it with another U2 pilot in it to talk the flying pilot down.

Good answer. There are several other considerations making the U-2 extremely hard to land compared to other craft. First, the wings produce a lot of lift, necessary to sustain flight at such a high ceiling. This produces a substantial "ground effect" where a cushion of air between the wings and ground makes touchdown difficult. In addition, the plane has bicycle landing gear and takes off with the aid of wingtip "pogo skis" that are jettisoned after takeoff, so the pilot must keep the plane very level on landing to avoid digging in a wingtip.

Lastly, the wide, straight wings reduce the speed range of the craft; too slow and you stall, too fast and you'll rip the wings off (especially at low altitude in the thicker air), so managing your glide slope is that much trickier.

Is landing the U2 much different than landing a sailplane?  Aside from the visibility, of course.

The U2 isn't much more than a jet-powered pressurized glider with some really expensive camera equipment aboard. It has a higher mass, so it needs a longer rollout than a sailplane, and a mistake would be more damaging as the sailplane would likely have a better strength-to-weight ratio on the wing spar (they're built for aerobatics, while the U-2's airframe likely wouldn't handle a barrel roll).

@KeithS - while pilots of course try to land as level as possible, the U-2 is designed to take wing strikes almost every time (and from what I've heard, usually does) with reinforced tips


text:
Can a lifting body work at supersonic speeds?
Can lifting bodies be made for supersonic flight, say up to Mach 3? What would they look like? How would they differ from lifting bodies for normal subsonic flight?

A lot of missiles generates lift from the body and  fly high speed.

AFAIK that's what they were first developed for, orbital reentry vehicles.

I didn't imagine lifting body flying at subsonic flight before reading this question. Then I realized they must slow down to land.

A lot of surface-air missiles uses lifting bodies now hence "wingless"

Lifting bodies were extensively used for reentry vehicles. In the Fifties and early Sixties there were two camps with different ideas how to go to space:

The Air Force wanted to increase vehicle speed and altitude incrementally such that finally a horizontally-launched, winged vehicle would reach space. Since the materials and propulsion were still lacking, they started at both ends of the flight envelope: One culminated in the X-15 research program, and the other researched how best to land a vehicle suitable for re-entry. This produced craft like the Northrop HL-10, which is shown below.

Northrop HL-10 with B-52 launcher overhead (picture source)

The US Army, on the other hand, tried to increase the size of rockets, traditionally used as an alternative to artillery, and got a helping hand from some other ex-Army guys who had lived through exactly the same split of technologies, where the Wehrmacht preferred rockets and the Luftwaffe preferred flying vehicles.

So not only can lifting bodies be made for Mach 3 and higher, they pretty much originated from the work done to fly at hypersonic speed, at least for a short while.

I don't know why a lifting body be not made supersonic. In supersonic regime, you just have to deal with shock waves and that a.c. theoretically moves to mid-chord region in 2-D analysis of the airfoil. In many practical cases, flow behind the shock wave happens to be subsonic,which makes it so much easy to apply your theories of subsonic physics over the behind shock wave region.

do you have any sources to back up your statements?


text:
How do turbofans and piston-engines compare in terms of maintenance costs?
I want to compare piston engines and turbofans on a maintenance basis. Let's say both engines are capable of 100 kN of thrust. Let's say both engines fly for 10,000 hours.
Over that time, what kind of maintenance can we expect from those engines. In particular:

How many times will the engine need major servicing?
How many man-hours does that servicing require?
About how much does it cost to do that servicing?

Major servicing for a piston engine would be something more than an oil change. For a turbofan it would be an overhaul.
Come to think of it, I don't know exactly what kind of servicing goes on for a piston engine other than an oil change. Is it like an overhaul where you dissasemble it and make sure everything fits together properly?
In case you're wondering, I'm envisioning these engines in a commercial cargo role. So no excessive dirtiness or hard throttling.

Most piston engines need an overhaul (complete tear down and rebuild) every 1800 to 2400 hours on average for GA engines. When you get into the thrust you are talking about you are really out of the typical range of piston engines and moving towards a turboprop. It is also hard to compare engines just using thrust numbers, piston engines rely greatly on prop design to produce thrust, so there is more than the engine to consider.

Is there any piston engine capable of 100 kN of thrust?

@vasin1987 The largest production piston, the Pratt & Whitney Wasp produced 3800 rated HP. The largest one made (2) was the Lycoming XR-7755, which produced 5000 HP. The GE90 produces 110,000 HP and outputs up to 510kn of thrust...I'm not sure that even with an ideal propeller setup that 100kn is possible out of a piston, living or dead.

@RonBeyer I would question the overhaul frequency that you cite. Those times may reflect the manufacturers recommended times between overhaul, but—legal requirements for certain operations notwithstanding—a well operated engine will exceed that time by 50-100% or more before a full teardown is required. At my old operation we routinely took engines to 3000-5000 hours and beyond before sending them in for rebuild, and then often merely as a precautionary measure, not out of need.

@JonathanWalters "need" wasn't the right word, I should have used "recommend", I wrote an answer on that (oddly enough you commented on :) ) a while ago that pretty much says what you said, I'll just chalk it up to it being late when I wrote the comment. But yes, you are right, some engines are very happy flying long beyond TBO.

@RonBeyer hah! I confuse between lbf and kN. Thanks for pointing that out.

Your question has some areas that depend on circumstances, models, etc. but to answer in general:

Piston engines are generally fully overhauled every 1200 to 2000 hours or so. Time to overhaul one varies with the engine model, and where the overhaul is done, as it can be done at the factory, at a repair shop, or by a qualified A&P/IA (a field overhaul). A piston engine overhaul can cost as much as $25,000 or more.
Turbofan engines will likely not require major service during those 10,000 hours. Most turbofan engines are inspected regularly and continue to operate on condition unless manufacturer recommended time periods are reached or exceeded. Time to overhaul varies greatly by engine model. Costs vary greatly as well, but in the case of turbofans you're talking in the hundreds of thousands of dollars.

Any clue why a turbofan overhaul would be much more expensive than a piston overhaul? AFAIK, overhaul is just disassembling all the parts, measuring them with laser precision, then putting them back together. Turbofan should just be unscrewing each blade from the disk. If anything, piston should be worse due to many special moving parts like the crankshaft, gearbox, those timing valves, carbeurator, etc. Maybe you were comparing a very large turbofan with a very small piston, since most pistons go on small GAs and most turbofans go on large commercial jets.

I'm not a specialist in this field, but I can tell you as someone who works on vehicle (piston) engines the main cost is labor and any "specialist" labor like repairing a transmission is very expensive.  Turbo fans are built to extremely tight tolerances that need a lot of modern tech which piston engines aren't. Yes piston engines require decent tolerance but there's a reason the term "backyard mechanic" exists.

@DrZ214 There is a difference between inspection, which is mostly labor, and the replacement of life-limited parts. Those parts are usually anything that moves/rotates. The law requires manufacturers to list which parts of the engine are life-limited (https://www.law.cornell.edu/cfr/text/14/33.70). The limits are not normally expressed in hours for turbine engines, they are based on cycles (starts). The cost of manufacturing those parts, as well as the labor to R&R them correctly, is what adds up to hundreds, if not millions of dollars, on some engines.


text:
Why are ethanol piston engines apparently cheaper to maintain than avgas piston engines?
In the course of hunting for examples of 100% ethanol-powered piston engines, I found the Embraer EMB 202 from Brazil that is modifiable to be that way.
But the article also says "The alcohol fuelled Ipanema engine also has 20% lower maintenance and operational costs." (no citation there, howerver.)
I also found this PDF that says similar things:

Avgas is slightly cheaper on a per-kilometre basis than ethanol, but the lower maintenance costs associated with the use of ethanol ultimately make it a cheaper fuel option for pilots.

Why is this? What exactly are the modifications made to normal engines that somehow reduces operation costs? And why aren't these modifications made to normal engines that just keep using avgas?
(Please: let's not get into an off-topic argument about whether ethanol should/will replace petrol products. I already know avgas is about 40 MJ/kg and ethanol more like 28. This question is not about that and I hope it doesn't become another casualty in that flame war.)

I suspect that a large part of the answer is because current piston engines were basically designed in the 1950s. Anything designed to run ethanol would take into account modern design techniques, like FADEC, fuel injection, and new materials.

Also, ethanol, unlike most avgasses, doesn't deposit lead on your sparkplugs.

Apparently a study was run for the FAA at Baylor University and it found that Ethanol caused less wear on the engines. You can trace the bibliography for quotes on the study. They concluded after testing for the O-235 engine

The relatively low wear and general engine cleanliness indicates that
  the engine can operate on ethanol fuel for longer time periods than on
  100LL avgas. An additional test is planned in which the engine will
  run according to the schedule met in this test, except for 300 hours
  rather than 150 hours, to justify an increase in recommended TBO for
  the engine operating on ethanol fuel.

The FAA also ran a wear test on E-85 that you can find here. Their summary is mainly about whether or not it works which it seems to, they don't touch very much on wear. Still worth taking a look at on the topic.
It does seem that ethanol can increase engine life as has been seen in current testing.


text:
Is tungsten a good metal for jet engines, besides the weight penalty?
Right now, the hottest part of jet engines are made from a steel-nickel alloy. Is tungsten a better choice?
Pure tungsten has the highest melting point and highest tensile strength of any other pure metal. Presumably you could make it hollow and use the same cooling fluid to prevent overheating. For this same reason I am not sure if the weight penalty is really that big. I don't think the turbine blades make up a really big part of total engine weight.
(I don't know how well it resists oxidizing at high temperatures. I don't know how it might change with adding a little carbon or steel or nickel. But presumably this is not a drawback because I've read about rocket engine nozzles using tungsten: link. If it doesn't oxidize or corrode there, I can't imagine why it would do so in a jet engine.)
Another way of asking this question: Does tungsten in the hottest part of jet engines have any major drawbacks besides weight and cost?

@ymb1 Yep, that's one reason I post it here. I really want to see what others have to say about that. The question is, is Tungsten a better choice than steel-nickel.

No, tungsten would generally not be a good choice for jet engines.
Some of the parts that would need to be the most heat resistant, such as turbine blades are subjected to rotational forces that increase directly with the mass of the part. Thus a high mass material such as tungsten would increase the forces the part would have to withstand. These forces are already measured in tons per blade.
If you discount the high-mass quality of Tungsten, the biggest problem is that Tungsten is also brittle. If a turbine blade failed, as does happen, it would be very difficult to contain. Tungsten is used in armor piercing projectiles; the results of a tungsten turbine blade failure would not be good.

I've read about rocket engine nozzles using tungsten: link. If it doesn't oxidize or corrode there, I can't imagine why it would do so in a jet engine.

Note these environments are 6 orders of magnitude apart in burn time, minutes vs tens of thousands of hours, driving very different requirements.

Welcome to Aviation.SE! While you make a very good point, this doesn't really answer the question. If you expand on it, you have potential for a good answer.

Yes the burn is shorter but far far more intense. The point was to say, if such intense burning (on a rocket engine) does not oxidize the tungsten (at least to the point of failure), then surely less intense burning (on a jet engine) will not either.


text:
Did WW2-era aircraft have constant-speed or variable-pitch props?
This is slightly related to my other question on variable-pitch controls.
I want to know if WW2-era aircraft typically had variable-pitch or constant-speed propellers. Variable-pitch lets the pilot adjust the propeller blade pitch. "Constant speed" is the same as variable-pitch, but the engine does it automatically to keep the best performance at various airspeeds. This also lets the engine keep a constant RPM so it can always run at its optimal RPM, also increasing efficiency.
Anyway, were such things seen in WW2? Were they the norm, or the exception? Did single-engine aircraft have them, or were they more common on multi-engines?
I'm interested in all the nations who made aircraft, especially USA, UK, USSR, Germany, and Japan. And I'm interested specifically in the combat aircraft, like fighters and bombers, not necessarily the civilian transports of the era. This will give us a good idea if the extra complexity and weight was acceptable for combat aircraft of the era.

i believe there was a change happening at the time. At the beginning of the war, there were many aircraft with fixed pitch. Some had high/low pitch. During WW2 technical innovation was rapid, and rather soon most high-performance aircraft had constant speed propellers. But not all aircraft were high-performance so some staid at fixed-pitch.

Most WW2-era military aircraft had constant speed propellers.
People often confuse variable pitch, controllable pitch, and constant speed. In general practice they are all the same thing, as all constant speed propellers are just the most common form of a variable pitch propeller.
There were very few aircraft where the pilot only had a simple control to just vary the pitch of the propeller. A few aircraft also had propellers that were only adjustable on the ground.
Most aircraft made since early WW II either had a fixed pitch, or constant speed propeller. “Constant speed” means the pilot controls engine RPM through either a hydraulically, or electrically, operated control system. It really is a type of RPM governor.
Some notable exceptions were the early Hurricane and Spitfire fighter aircraft that initially had wooden, 2-bladed, fixed pitch propellers because they were lighter than metal constant speed propellers. There were performance penalties though so these were soon replaced with metal 3 bladed, 2 position "controllable" propellers, with coarse and fine pitch settings. By 1940, they were all updated to metal constant speed propellers.

..." In general practice they are all the same thing as all constant speed propellers", this is not true. The difference between variable pitch and constant speed is the variable pitch DOES NOT HAVE A GOVERNOR. The Continental "E" series engines found on Bananza, Navion, Grummen Goose, etc are all variable pitch (no governor). There was an adapter called a "T" drive that could be added that would convert the "E" series to constant speed. I owned several Navions and pilot workload with a variable pitch is considerably higher than a "set and forget constant speed".

The focus of the question was WW II military aircraft. Even for general Aviation aircraft, those aircraft you mention are exceptions, and my statements "in general" and "most common" are still true.

The Navion and Grumman Goose ARE WWII Military aircraft (and there are others). There is no "general practice" where variable pitch is "the same" as a constant speed.

@jwzumwalt, The Navion is not a WWII military aircraft in any sense of the term.  According to http://www.navion.com/history.html it was "designed at the end of the WWII... for the civilian market."  Having a few models also designated for military use later in that same decade doesn't gain it acceptance into the big boy club that won the war.  (the OP emphasized they were interested specifically in fighters and bombers)

@ Michael Hall - I owned two military Navions it depends on the model (The original A model were mostly military).  North American, built 164 improved L-17B Navions for the U.S. Air Force. General McCarthy was assigned one and flew it during the Philippine campaign. See this article by the National Military Museum. https://www.nationalmuseum.af.mil/Visit/Museum-Exhibits/Fact-Sheets/Display/Article/196106/north-american-l-17a-navion/

Variable pitch (with feathering) was virtually universal on (at least American) multi-engine fighters, bombers, and transport aircraft even early in the war: the DC-2 had feathering variable pitch, and it entered service in 1934.  The P-38 had feathering and variable pitch, and was in service before the US entered the war.
Constant speed propellers were also found on many single engine combat aircraft -- one was the Hellcat, which entered service in 1943, though it's very possible the technology was introduced earlier.  The P-47 Thunderbolt also had this feature.
When you're talking about aircraft that weigh several tons and need to deliver thousands of horsepower through a propeller, especially for combat operations, the weight and cost of variable or constant-speed propellers became negligible by comparison to the cost of the aircraft or the cost of fielding a losing design.

You haven’t really answered the question clearly and seem to be adding to the confusion . Both the P-38 and DC-2 had constant speed propellers, as did most high performance aircraft of the time.

A constant speed propellor is a variable pitch propellor, the idea is that the pitch is adjusted automatically so that engine rpm remains constant with varying altitude and power settings.

The answer is yes, they were used in World War Two, although some light aircraft and older types used in the early war years had fixed pitch propellors.

This defines what a constant speed prop is, but it doesn't seem to answer the question about whether they were used in WW2 or not

The answer is yes, they were used in World War Two, although some light aircraft and older types used in the early war years had fixed pitch propellors.

A variable pitch propeller does NOT have a governor while a constant speed does. The two are not the same.

A constant speed propellor has variable pitch and a governor, whereas a variable pitch propellor does not necessarily have a governor.

Constant speed propellers were introduced in WWII.  If you have a somewhat late war perspective (USA), then they were ubiquitous.
However, there is a great story about coming in and retrofitting Spitfires as quickly as possible during the war to upgrade them to constant speed props.